How much would this improve my machine?

I plan on doing some upgrades later this year, near winter. I have a E6E00 now OC'd to 3200, Radeon 5700 HD and 4 GB RAM DDR2.

I wanted to buy Phenton II X4 Quad Core 840 and 4 GB RAM DDR3, plus motherboard for it (maybe upgrade to 8 GB in future).

Would it be a big improvement? It's the best and cheapest CPU I can find at 400 PLN mark (a bit over $130), and motherboard + RAM cost about 200 together (100 for memory, 120 for the rest). Don't have to change PSU, as the new CPU would require the same amount of power.

The next best thing would cost me at least 50% more. Still a lot cheaper than Intels in Poland (just to compare - similar power Intel Core Quad would cost at least 600-700 PLN).

Since the (Pentium) E6E00 is a misprint, and I can't find a Phenom x4 840 on this list:
http://www.tomshardware.com/reviews/...ce,3007-5.html
I can't say for sure, but I would expect that, no, it's not that great of an upgrade.

E6300 is in the list. Finger slipped and instead of 3 wrote E.

"Pentium Dual-Core E5700, E5800, E6300, E6500"

And I'd assume that X4 840 is somewhere:

Phenom II X4 945, 940, 920, 910, 910e, 810

Here, at least judging by stats. Hell, scroll down, you see "Best prices for tested products":

AMD Phenom II X4 840 3.20.

Weird that they would omit it.

If you read the whole article that that list is part of, they recommend that an upgrade is not worthwhile unless you can go up about two groups - which is also what I would say.

You seem to have also done a finger slip on the graphic card as well. Is it HD5750 or HD5770? (If it's actually an HD5700, that would probably be a low-end OEM-only product, but I can't find any reference to it, other than just "HD5700 family".)

In either case, you won't do much of an upgrade without going a bit further up the list - and spending more money. If your graphics card is an HD5750, your best upgrade might be to go to an AMD HD6850 or an nVidia GTX 460 (1Gig) based, video card.
Check the "Best gaming CPU" and "Best gaming graphics" articles on Tom's Hardware site for an idea of what would be a worthwhile upgrade.

There is no HD5700 but many tools report HD5770/5750 as HD5700 series.

Pretty gosh darn shure that it's a 5770. Unless the box is lying, my drivers are lying and my PC is lying me. And the fine print on the GPU itself.

I would recommend against the lateral from an Intel E6300 to AMD Phenom II x4 840. Changing to an Phenom II x4 will not improve your game play. DDR2, really? I would advise you save your money and do a major upgrade or at least go with a DDR 3 MoBo that meets your needs.

Yes, I was going to buy DDR3. I have DDR2 now.

I do consider memory, motherboard AND CPU to be a major upgrade, everything except for GPU would be new.

edit: How would AMD Phenom II X4 955 or 965 sound? They cost relatively little (60 and 100 pln more respectively), and are (according to the list in link above) 3 levels above my current CPU.

That would be a much more worthwhile upgrade.